In this study, we have examined the effects of estradiol and progesterone on pulsatile luteinizing hormone (LH) secretion in the rat. In the first experiment, regularly cycling rats were ovariectomized on metestrus and a Silastic implant containing estradiol in oil (7.5, 15 or 30 micrograms/ml) or crystalline progesterone (0.7 or 3 cm long) was placed s.c. immediately after surgery. Rats were cannulated 3 days later and frequent blood samples collected (every 10 min for 4 h) the next day. Estradiol treatment alone produced a dose-dependent suppression in mean LH by decreasing LH pulse amplitude without altering interpulse interval. Progesterone treatment alone had no effect, but when the low progesterone dose was combined with a subthreshold estradiol treatment, the combination decreased LH pulse amplitude, but had no effect on the interval between pulses. The combination of high progesterone and low estradiol completely suppressed LH secretion. In the second experiment, rats were bled 7 days postovariectomy to increase the postcastration LH rise and examine higher steroid treatments. Three progesterone treatments (0.7, 1.5 and 3 cm long implants) were combined with the low estradiol treatment; the high estradiol dose (30 micrograms/ml) was also tested. All treatments decreased LH pulse amplitude without altering interpulse interval, although the highest progesterone treatment again completely suppressed LH in most rats so that no statistical analysis could be performed. No firm conclusions can be drawn about the effects of the high steroid treatments that completely suppressed LH; such an effect could reflect either inhibition of pulse frequency or amplitude.(ABSTRACT TRUNCATED AT 250 WORDS)